The FBI has received a “plausible but unsubstantiated” report that al-Qaeda terrorists may have discussed attacking the New York City subway system during the Thanksgiving holiday weekend, US authorities said today.

Russ Knocke, spokesman for the Department of Homeland Security, said that the FBI warning to New York law enforcement agencies meant that there may be an increased police presence in New York and other large metropolitan areas this weekend.

The report indicated that al-Qaeda terrorists “in late September may have discussed targeting transit systems in and around New York City”.

“These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger rail systems,” the leaked memo, dated yesterday, said. “We have no specific details to confirm that this plot has developed beyond aspirational planning, but we are issuing this warning out of concern that such an attack could possibly be conducted during the forthcoming holiday season.”

The memo was notable for its blunt language. Intelligence and homeland security officials are working with local authorities to try to corroborate the information “and will continue to investigate every possible lead,” the memo said.

Mr Knocke added that the warning was issued “out of an abundance of caution going into this holiday season”. No changes were being made to the nation’s threat level, or for transit systems at this time, he said.

“However, transit passengers in larger metropolitan areas like New York may see an increased security presence in the coming days. This includes uniformed and plain clothed behaviour detection officers, federal air marshals, canine teams, and security inspectors,” he said.

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